1. Field of the Invention
The present invention pertains to keratometers for measuring the roundness of the cornea of an eye and, more particularly, to a hand-held keratometer for projecting a circle of light on the cornea to measure roundness thereof.
2. Discussion of the Prior Art
After ocular surgery, it is important that the cornea remain round to avoid astigmatism in that, if the cornea is round, no correction for astigmatism need be incorporated in glasses for the patient thereby reducing the thickness and distortion of the glasses as well as the expense thereof. The problem of leaving the eye non-astigmatic has been increased with the acceptance and increased frequency of intraocular lens implant surgery in that, by eliminating astigmatism and choosing the proper power implant, a patient having intraocular lens implants will not require glasses for distant or near vision, whichever is chosen. There are only two structures in an eye having an intraocular lens implant therein that can cause astigmatism, one structure being the intraocular lens, which is manufactured to be non-astigmatic, and the other structure being the cornea, which can be manipulated to be non-astigmatic. Accordingly, astigmatism commonly associated with the natural lens in an eye is not a problem in an eye having an intraocular lens implant. Thus, the wide acceptance of intraocular lens implant surgery has greatly increased the interest in and importance of controlling the shape of the cornea as surgery is concluded since the cornea is the only source of astigmatism remaining in the eye. With astigmatism controlled, cataract surgery with insertion of an intraocular lens implant of proper power in place of the natural lens can result in clear vision without glasses whereas, without astigmatism control, such surgery can be expected to usually produce only clear vision with glasses.
To recognize astigmatism of the eye, usually a circle of light is held above the eye, and the observer looks at the reflection of the circle of light on the cornea. If the reflected light is a perfect circle, the eye is round and non-astigmatic. If, however, the reflected light is an oval, the cornea is not round and is astigmatic. The shape of the oval and the positioning of the axis of the oval is indicative of the amount and axis of the astigmatism.
Astigmatisms are usually corrected by tightening or loosening one or more of the sutures that close the cornea-scleral wound made for removal of the cataract. It has been found that tightening a suture flattens the peripheral cornea and steepens the central cornea in the visual axis. For example, if the reflection from the cornea of the circle of light was an oval with its longest dimension or axis in the vertical direction, this would indicate the cornea was flattened in the 12-6 o'clock axis. The 12 o'clock suture would then be tightened to make the cornea round. Pieces of cornea tissue can be removed, sutures loosened, and radial incisions can also be made to correct cornea astigmatism.
Prior art devices for forming a circle of light to be projected on the cornea for detecting astigmatism include surgical keratometers designed to rest on the eye in ocular contact, keratometers permanently mounted on operating microscopes and using fiber optics to produce spots of light arranged in a circle with a grid for evaluating roundness, and keratometers permanently mounted on operating microscopes wherein the reflection of the circle of light from the cornea is evaluated with prism images and a microcomputer provides a digital readout of the amount and axis of astigmatism. Such prior art keratometers suffer the disadvantages of requiring contact with the eye, of being extremely expensive and/or of being of a complex design and difficult to manipulate and operate.
Another proposed device is formed of a clear glass ring held by a stainless steel rim with a stainless steel handle to provide a hand-held, non-ocular contact keratometer. This device suffers the disadvantages of being relatively expensive to produce due to the separate manufacture of the glass ring and the stainless steel rim requiring mounting together, of the glass ring being fragile and breakable if dropped, and of requiring a light source to be disposed directly above the ring to provide coaxial illumination such that the ring of light produced by the device is evenly illuminated. If incident light is received obliquely by the glass ring, much of the incident light will be blocked by the portion of the stainless steel rim nearest the light resulting in reduced light being emitted from that side of the glass ring. On the side of the glass furthest from the light, the stainless steel rim does not block the light and actually reflects additional light into the glass ring such that the circle of light directed to the eye is uneven creating difficulty in determining the extent of any reflected oval. Additionally, frosted areas of the glass ring where it is cut on the inside and the outside, causes reflection of a circle of light appearing to be a double ring furthest from the light and a single ring nearest the light with oblique illumination. This device, accordingly, has the disadvantage of being useless, from a practical standpoint, when obliquely illuminated, it being noted that surgical loupes and many operating microscopes are obliquely illuminated.